The present invention relates to an ethylene copolymer exhibiting excellent transparency, impact resistance and opening property when it is formed into a bag (hereinafter merely referred to as opening property).
High-pressure polyethylene is known as a resin having a relatively high transparency. In order to make the best use of its transparency, high-pressure polyethylene is used as a material for films, hollow containers and the like. However, as far as the film application is concerned, a thin film made of high-pressure polyethylene is useless for practical purposes because it exhibits low tear strength and impact strength. Therefore, high-pressure polyethylene is limited in its film application.
On the other hand, it is known that a copolymer of ethylene with an .alpha.-olefin having 3 or more carbon atoms, which is prepared by copolymerizing the ethylene with the .alpha.-olefin in the presence of a Ziegler catalyst or a Phillips catalyst under a low pressure, is a resin having an excellent mechanical strength and almost the same density as that of the high-pressure polyethylene. Ordinarily, in the production of the copolymer, titanium type catalysts are used as the Ziegler catalyst, or chromium type catalysts are used as the Phillips catalyst, and a pressure of 100 kg/cm.sup.2 or less is used. The copolymers resulting from these processes are disadvantageous in that they exhibit poor transparency and opening property.
In this connection, there has been proposed a process for improving the transparency of the copolymers as disclosed in Japanese Patent Application Laid-open No.53-92887. However, as far as we know, the copolymer obtained by such a process is still unsatisfactory in its opening property.
The opening property is one of important qualities for a packaging film. If a bag has a poor opening property, it is impossible to pack efficiently an article thereinto. With regard to polymeric films, the index of blocking is used to indicate the adhesion of the polymeric films to each other. The index "opening property" as used herein signifies nothing but blocking when the polymeric films are in a specific form of product, i.e., bags.